The documents advise locking doors, turning off lights and remaining quiet and calm.

"We must also ensure that unarmed security personnel ... do not attempt to become part of the response, but could be invaluable to the evacuation efforts," the documents said.

In addition, a training video shown to aviation police officers has the same instructions: "If evacuation is not possible, you should find a place to hide where the active shooter is less likely to find you. Block entry to your hiding place and lock the door."

That makes no sense, said aviation police officers interviewed by CNN.

Aviation officers say they want to be able to carry guns on the job.

"We're not trying to replace the Chicago police officers; we just want to have the tools to do the job like every other law enforcement agency in the country," said one officer who requested anonymity.

"We're nothing but casualties if you tell us to run and hide. And how can the public look at us if they see police officers running and hiding? That goes against the very oath we were sworn to that we took."

"And that's amazing these men and women are sent to the Chicago police academy, and trained as police officers, and being a former police officer, I know your first instinct is to go to the problem -- not run away from the problem," Brandon said.

He said the union has been unable to get the aviation department to change the no gun policy, which dates to the early 1990s.

"They are the last resort to the airfield in many cases, and to have them unarmed is just, I think, it's ludicrous," he said.

Asked if this boiled down to a union issue, Brandon said he was willing to give up the aviation police officers as union members if it meant allowing them to carry guns.

"I want their safety as the first thing, and their ability to respond to threats is number two," he said.

"I've never heard of anything so crazy," Black said. "I mean, the concept in a post-9/11 world of having sworn law enforcement officers unarmed at an airport, not being able to take direct action -- I don't know what they are thinking."

"Who in their right mind would have sworn law enforcement officers wearing a sign on their back that says police and wearing a badge on their chest and being unarmed?" he asked.

Chief Richard Edgeworth, who oversees the aviation police officers, did not respond to multiple calls from CNN. After being approached by a CNN reporter outside his office, he briskly walked away, saying "no comment."

"We think the strategy in place is working," said Owen Kilmer, deputy communications director of the Chicago Aviation Department.

In a statement to CNN, Kilmer wrote that with "the current security structure in place, violent crime incidents at O'Hare and Midway airports are extremely low -- ensuring that the millions of passengers who fly through Chicago each year feel secure at the airports."

"The multilevel security approach used at O'Hare and Midway Airports has proven effective in stopping and preventing crime while creating a structure that allows for all our law enforcement officials to collaborate easily and effectively."

Kilmer wrote, "There are different security structures in place at major airports across the country, and there's no one-size-fits-all approach."

Citing "safety concerns," the aviation department would not respond to questions about its policy on active shooters.

And while the aviation department would not discuss staffing levels, the city's 2015 budget appropriation records show about 309 positions for aviation police officers and supervisors. The union says, due to vacancies, there are about 279 actively on the job.

The budget also shows 231 armed Chicago police officers and supervisors at O'Hare and Midway. Chicago police spokesman Anthony Guglielmi says that staffing level is sufficient. He also said arming or not arming aviation officers is not his department's issue.

"For whatever reason the governing agency over these officers decided not to arm them," he said.

He said, "The aviation security officers are designed to be a force multiplier."

A CNN survey of major airports shows, for the most part, similar staffing levels of armed police.

A CNN survey of major airports shows, for the most part, similar staffing levels of armed police.

For example comparing the top three airports in the United States in terms of passenger volume, Atlanta's Hartsfield-Jackson International Airport, the busiest, has 178 armed police and 21 nonsworn officers. The Atlanta airport declined to provide specific numbers on additional security, which includes unarmed aviation security personnel and private security guards.

Other major U.S. airports, such as those in New York, Washington, Dallas-Fort Worth, Seattle and Phoenix, use unarmed security guards for added surveillance and other functions, but these guards are not sworn police officers, according to spokespersons for the airports.

Duane McGray, executive director of the Airport Law Enforcement Agencies Network, said, "All airports are required to do is provide sufficient police resources" and "the numbers vary airport to airport."

The gun issue is part of an ongoing dispute between the aviation police officers and Edgeworth. In September, the officers took a vote of no confidence in him.

The no confidence letter from the union states, "The overwhelming majority of Aviation Police Officers and this Union agree that Chief Edgeworth is incompetent as a leader of police, noncommunicative, ineffective and exerts control through coercion and fear in his position as chief overseeing public and personnel safety."

In a response to the union, Chicago Aviation Department Commissioner Ginger Evans wrote that Edgeworth has the "full confidence and trust" of the department and "has been a tireless advocate for the Safety and Security section throughout his tenure."

"Both Midway and O'Hare have exemplary safety and security records and are recognized as leaders in the aviation industry," Evans wrote.